1. Field of the Invention
This invention relates to photovoltaic devices and is particularly relevant to the area of solar cells.
2. Art Background
Various methods including chemical vapor deposition (CVD) techniques have been used to make p III-V/n II-VI, heterodiodes by depositing a II-VI semiconductor on a III-V semiconductor substrate. For example, Ito and Oshawa have reported fabricating an n-CdS/p InP single crystal device by using a hydrogen-cadmium sulfide vapor growth technique. (See Jap. J. Appl. Phys., 14 (8), 1259 (1975).) Mack et al have used a hydrogen-hydrogen chloride growth technique to etch a GaAs substrate and to deposit a ZnSe layer. (See Phys. Stat. Sol., 2, 701 (1970).)
The efficiency of a particular type heterodiode or even the ability to fabricate a heterodiode device depends on which of the various techniques is chosen to make the device. The p InP/n CdS cell is an apt illustration. Efficiencies up to 14% have been reported for single crystal p-InP/n-CdS devices made by depositing CdS on an InP substrate through molecular beam epitaxy followed by an annealing step. (See co-pending U.S. application of Bachmann et. al., Ser. No. 587,042, filed June 16, 1975 now U.S. Pat. No. 3,988,172, (Canadian application Ser. No. 244,052 filed Jan. 22, 1976); see also co-pending U.S. application of Bachmann et al Ser. No. 672,878, filed Apr. 2, 1976 both of which are hereby incorporated by reference.) Yet, single crystal devices made by a CVD hydrogen transport technique have been reported to have only a 4% efficiency, (see Ito and Oshawa, Jap. J. Appl. Phys., 14 (8), 1259 (1975). This demonstrates the desirability of a method involving a minimum number of steps which yields consistently high efficiencies (on the order of 13%). Particularly desirable is a CVD process with these attributes because of the adaptability of CVD techniques to large scale production.
While in the InP/CdS system efficiencies vary with the process employed, fabrication of some other prospective n-CdS/p-III-V heterodiode systems with useful efficiencies to date have not been reported. For example, an n-GaAs/n-CdS has been made by a close spaced chemical vapor deposition technique. (See Jap. J. Appl. Phys. 14 (16), 1547 (1975).) However, in the same article a p-GaAs/n-CdS device was shown to have very poor diode characteristics. Thus, it would also be advantageous to have a process which allows fabrication of such a device with diode properties.